No. 3-] THE DISCODRILID NEPHRIDIUM. 353 



in great part deeply buried in the plexus mass, where it is very 

 conspicuous in sections (Figs. 14, 15). 



Considering once more the entire extent of the simple tubule, 

 we find it made up of greatly elongated tubular (drain-pipe) 

 cells, each with a nucleus placed at about its middle, and in 

 many cases with a group of cilia at the same point. We can- 

 not doubt that the diverticula of the lumen indicate the termi- 

 nals of conjoined cells, and possibly they are indicative of 

 imperfect union, though developmental facts do not support 

 this view. Oka's recent figures of the nephridium of Clepsine 

 tesselata show that the lateral branchings of the lumen of the 

 main lobe are developed at points corresponding to the bound- 

 aries of conjoined cells (Zeit. f. wiss. ZooL, LVIII (1894), pp. 

 79-151). Measured by this standard the cells will be seen to be 

 longest in the long tubule loop, where they measure about 

 .18 mm., which is perhaps the longest simple drain-pipe cell 

 that has been described. They are shortest in the coelomic 

 efferent region, and the short tubule loop, where they measure 

 about .1 mm. 



The plexuses of the plexus region are to be compared with 

 the nodal diverticula of the tubule region, which is shown by 

 their position and mode of development ; they begin as simple 

 diverticula which enlarge, become divided, branch, and anasto- 

 mose during the later development of the worm. In both 

 regions the nuclei and cilia are similarly placed. The compari- 

 son will be clear on consulting the diagrams (Figs. 3, 3^). 



7. The Terminal Vesicles. 



In all Discodrilidae, so far as known at present, the efferent 

 tubules of the posterior nephridia terminate in separate small 

 vesicles which communicate with the exterior by pores. The 

 same is true of the anterior nephridia of B. parasita, B. insta- 

 bilia, and B. pulcherrima. But the American genera Bdello- 

 drilus and Pterodrilus present the unusual condition in annelids 

 of a single median vesicle common to both. Bolsius (9) has 

 described a median ventral opening common to a pair of poste- 

 rior nephridia of Mesobdella; and Vejdovsky (34), a median 



